Transverse cracks in the head of a rail generally develop in the median axis of the rail, as shown in FIG. 1. These well known cracks are called oval flaws.
These defects arise during manufacture, wherein a non homogenous core can, later on, give rise to a small crack which then propagates and gets bigger.
Rails on which trains travel undergo enormous stresses that enlarge these cracks which can extend over 100% of the rail cross-section, but ruptures of the rail, at the location of these cracks, may occur with cracks having a cross-sectional area as little as 20% of the rail, according to the circumstances (track, traffic).
Detection of these cracks is made by ultrasonic techniques. An ultrasonic beam having a refracted angle within the steel of 70.degree. is directed toward the core of the head of the rail from a transducer placed on the rolling table of the rail. The energy which is reflected by the crack is detected and then measured by electronic circuits.
The operator can then take the necessary measures as a function of the significance of the detected cracks. He may observe the rail in question if it has small cracks or ask for its replacement in the case of big cracks.
Transverse cracks displaced either right or left of the median part of the head of the rail on either side of its symmetry axis, as shown in FIG. 2, have been known for several years.
Other transverse crackings propagate from the upper left and right corners of the head of the rail (FIG. 3).
The origin of these displaced cracks may be due to the type of rail itself, to the load per rolling axle of the trains and to the evolution of the steels used for the manufacture of the rails.
In certain countries, the phenomenon of FIG. 2 is of great occurrence and in other countries it is that of FIG. 3 which occurs most often.
In each of these cases there is the problem of the detection of displaced oval flaws.
At present detection of these displaced oval flaws (FIGS. 2 and 3) cannot be effected with precision and certainty. As a matter of fact, using the known methods, which were developed for the detection of the centered oval flaws (FIG. 1) one can never be sure of the nature of the flaws: for a given echo, was it reflected by a centered oval flaw of a certain amplitude or by one or several displaced oval flaws, of a different magnitude? This ambiguity in the measurement cannot be dealt with by the means now available and leads to a great risk of rupture or to the premature replacement of rails if one does not want to take any risks.
Furthermore, as seen in FIG. 4, the surface of the head of the rail is ordinarily deformed at its sides through the wearing off caused by the rolling of trains. Therefore, it is not possible to use ultrasonic emitting transducers which rest against the edges B and C of the head of the rail in order to try to detect the displaced oval flaws, because as the contact with the rail is not adequately realized in these places, the precision of the measure would be insufficient due to the fact that one would not obtain a good acoustic contact due to the shapes of the transducer and of the rail and therefore the trajectory of the ultrasonic beam in the rail would not be known with precision.